As a mechanism which converts rotary motion to linear motion, a ball screw device is known. The ball screw device includes a linear screw shaft on which helical rolling grooves are formed on the outer circumferential surface, and a nut on which helical rolling grooves opposing the rolling grooves of the screw shaft are formed on the inner circumferential surface, the nut is screwed with the outer circumferential side of the screw shaft via a plurality of balls interposed between both rolling grooves, and is able to move in an axial direction of the screw shaft by rotating the screw shaft. Seal members are provided on both ends in the axial direction of the nut, each of the seal members is externally fitted to the nut, prevents intrusion of dust or the like from the outside to a portion between the nut and the screw shaft, and prevents leakage of lubricant existing between the nut and the screw shaft to the outside.
Since the rolling groove is formed on the outer circumferential surface of the screw shaft, across-sectional shape of the screw shaft in a direction perpendicular to the axial direction is not a simple circle. Accordingly, a gap is easily generated between the screw shaft and the seal member due to errors of shape accuracy or mounting errors of the seal member, and in order to prevent this, heavy labor is required for control and adjustment of the shape and mounting accuracy of the seal member. If interference of the seal member with respect to the screw shaft is increased, the errors of shape accuracy or mounting errors are reduced, and the occurrence of the gap is able to be suppressed. However, simultaneously, a contact pressure of the seal member with respect to the screw shaft is increased. That is, friction resistance of the seal member with respect to the screw shaft is increased. Accordingly, there is a concern that operating performance of the ball screw device may be deteriorated. Moreover, since heat due to the friction is increased, positioning accuracy of the ball screw device is decreased.
Thus, a ball screw device is suggested in which a lip portion of a seal member is composed of a lubricant-containing polymer, and a thickness dimension of the lip portion is smaller than a width dimension of a rolling groove of a screw shaft (for example, refer to PTL 1). According to the configuration disclosed in PTL 1, even when predetermined interference with respect to the screw shaft is formed on the seal member, due to a decrease in a contact area between the lip portion and the screw shaft and a friction reduction effect by the lubricant exuding from the lubricant-containing polymer, the friction resistance of the seal member with respect to the screw shaft is able to be suppressed.